U.S. patent application number 10/825062 was filed with the patent office on 2005-10-20 for combined laser altimeter and ground velocity measurement apparatus.
Invention is credited to Halama, Gary E., Jamieson, James R., Meneely, Clinton T..
Application Number | 20050231710 10/825062 |
Document ID | / |
Family ID | 35013368 |
Filed Date | 2005-10-20 |
United States Patent
Application |
20050231710 |
Kind Code |
A1 |
Jamieson, James R. ; et
al. |
October 20, 2005 |
Combined laser altimeter and ground velocity measurement
apparatus
Abstract
Combined laser-based apparatus for determining both altitude and
ground velocity of an aircraft comprises: a laser source for
emitting pulsed laser beams substantially at a predetermined
wavelength; a plurality of first optical elements for directing the
laser beams from a first optical path to a second optical path
which exits the first optical elements; a plurality of second
optical elements configured to form a telescope, the second optical
path and telescope field of view being fixedly co-aligned; an
optical scanner for directing the second optical path and telescope
field of view to desired ground positions while maintaining the
co-alignment thereof; the telescope for receiving Doppler
wavelength shifted reflections of the pulsed laser beams and
directing the received ground reflections substantially over a
third optical path; an optical filter element for separating the
ground reflections of the third optical path into first and second
portions that are dependent on the Doppler wavelength shift of the
ground reflections; and processing means for determining altitude
and ground velocity of the aircraft based on the first and second
portions.
Inventors: |
Jamieson, James R.; (Savage,
MN) ; Halama, Gary E.; (Burnsville, MN) ;
Meneely, Clinton T.; (Burnsville, MN) |
Correspondence
Address: |
CALFEE HALTER & GRISWOLD, LLP
800 SUPERIOR AVENUE
SUITE 1400
CLEVELAND
OH
44114
US
|
Family ID: |
35013368 |
Appl. No.: |
10/825062 |
Filed: |
April 15, 2004 |
Current U.S.
Class: |
356/28 ;
356/28.5 |
Current CPC
Class: |
G01P 3/366 20130101;
G01S 7/4812 20130101; G01S 17/58 20130101; G01S 17/87 20130101;
G01S 17/933 20130101; G01S 7/4817 20130101; G01C 5/005
20130101 |
Class at
Publication: |
356/028 ;
356/028.5 |
International
Class: |
G01P 003/36; G01N
021/00 |
Claims
1. Combined laser-based apparatus for determining both altitude and
ground velocity of an aircraft, said apparatus comprising: a laser
source for emitting pulsed laser beams substantially at a
predetermined wavelength over a first optical path; a plurality of
first optical elements for directing said laser beams from said
first optical path to a second optical path which exits said first
optical elements; a plurality of second optical elements configured
to form a telescope with a predetermined field of view, said second
optical path and telescope field of view being fixedly co-aligned;
an optical scanner disposed in said second optical path for
directing said second optical path and telescope field of view to
desired ground positions while maintaining the co-alignment
thereof; said telescope for receiving from said desired ground
positions Doppler wavelength shifted reflections of said pulsed
laser beams within the field of view thereof and directing said
received ground reflections substantially over a third optical
path; an optical filter element disposed in said third optical path
for separating the ground reflections of said third optical path
into first and second portions that are dependent on the Doppler
wavelength shift of said ground reflections; and processing means
for determining altitude and ground velocity of said aircraft based
on said first and second portions.
2. The apparatus of claim 1 wherein the processing means includes:
a first light detector for receiving and converting said first
portion of ground reflections into first electrical signals
representative thereof; a second light detector for receiving and
converting said second portion of ground reflections into second
electrical signals representative thereof; and a processor for
determining the ground speed of the aircraft at each ground
position based on a function of the first and second electrical
signals.
3. The apparatus of claim 2 wherein the processing means includes a
means for determining a laser beam ground scan vector of the
scanner for each ground position; and wherein the processor is
operative to associate the ground speed with the corresponding
ground scan vector for each ground position.
4. The apparatus of claim 3 wherein the processor is operative to
determine ground velocity using the ground speeds and corresponding
ground scan vectors of at least three ground positions.
5. The apparatus of claim 4 wherein the processor is operative to
determine ground velocity by a triangulation of the ground speeds
and corresponding ground scan vectors of the at least three ground
positions.
6. The apparatus of claim 2 wherein the processor is operative to
determine the ground speed of the aircraft at a ground position
based of a ratio of a difference over a sum of the first and second
electrical signals corresponding to the ground position.
7. The apparatus of claim 1 wherein the optical filter element is
operative to transmit the first portion of the ground reflections
of said third optical path therethrough and to reflect the second
portion of the ground reflections of said third optical path to a
fourth optical path, said transmission and reflection of the first
and second portions by the optical filter element being dependent
on the Doppler wavelength shift of said ground reflections.
8. The apparatus of claim 7 wherein the optical filter element has
a sharp cut off transmission response with respect to wavelength
such that a small Doppler shift in wavelength away from the laser
emission wavelength will produce a detectable change in
transmission characteristics of the optical filter element.
9. The apparatus of claim 8 wherein the optical filter element is
tuned to receive the laser emission wavelength along a cut off edge
of the transmission response.
10. The apparatus of claim 9 wherein the optical filter element is
tuned to receive the laser emission wavelength at approximately
midway of the cut off edge of the transmission response.
11. The apparatus of claim 7 comprises a dichroic beam
splitter.
12. Laser-based apparatus for generating signals for use in
determining both altitude and ground velocity of an aircraft, said
apparatus comprising: a laser source for emitting pulsed laser
beams substantially at a predetermined wavelength over a first
optical path; a plurality of first optical elements for directing
said laser beams from said first optical path to a second optical
path which exits said first optical elements; a plurality of second
optical elements configured to form a telescope with a
predetermined field of view, said second optical path and telescope
field of view being fixedly co-aligned; said telescope for
receiving Doppler wavelength shifted reflections of said pulsed
laser beams within the field of view thereof and directing said
received reflections substantially over a third optical path; an
optical filter element disposed in said third optical path for
separating the reflections of said third optical path into first
and second portions that are dependent on the Doppler wavelength
shift of said reflections; and light detection means for receiving
said first and second portions and generating first and second
signals representative of said first and second portions,
respectively.
13. The laser-based apparatus of claim 12 wherein the laser source
is autonomously operative to periodically generate laser pulses;
and wherein the light detection means is operative to generate the
first and second signals corresponding to each laser beam
reflection.
14. The laser-based apparatus of claim 13 including a means for
generating a pulse signal representative of a start of each laser
pulse period.
15. The apparatus of claim 12 wherein the optical filter element is
operative to transmit the first portion of the reflections of said
third optical path therethrough and to reflect the second portion
of the reflections of said third optical path to a fourth optical
path, said transmission and reflection of the first and second
portions by the optical filter element being dependent on the
Doppler wavelength shift of said reflections.
16. The apparatus of claim 15 wherein the optical filter element
has a sharp cut off transmission response with respect to
wavelength such that a small Doppler shift in wavelength away from
the laser emission wavelength will produce a detectable change in
transmission characteristics of the optical filter element.
17. The apparatus of claim 16 wherein the optical filter element is
tuned to receive the laser emission wavelength along a cut off edge
of the transmission response.
18. A distributed laser-based system for use on-board an aircraft
for determining both altitude and ground velocity of said aircraft,
said system comprising: at least three laser-based measurement
apparatus for disposition at different locations on said aircraft,
each said apparatus comprising: a laser source for emitting pulsed
laser beams substantially at a predetermined wavelength over a
first optical path; a plurality of first optical elements for
directing said laser beams from said first optical path to a second
optical path which exits said first optical elements; a plurality
of second optical elements configured to form a telescope with a
predetermined field of view, said second optical path and telescope
field of view being fixedly co-aligned; said telescope for
receiving Doppler wavelength shifted reflections of said pulsed
laser beams within the field of view thereof and directing said
received reflections substantially over a third optical path; an
optical filter element disposed in said third optical path for
separating the reflections of said third optical path into first
and second portions that are dependent on the Doppler wavelength
shift of said reflections; and light detection means for receiving
said first and second portions and generating first and second
signals representative of said first and second portions,
respectively; each said laser-based apparatus configurable to
direct its co-aligned second optical path and telescope field of
view from said aircraft to a different ground position from the
other laser-based apparatus; and a processing unit for receiving
and processing said first and second signals from said at least
three laser-based apparatus to determine both said altitude and
ground velocity of said aircraft.
19. The system of claim 18 wherein the processing unit is operative
to determine a ground speed of the aircraft for each laser-based
apparatus based on a function of the corresponding first and second
signals generated thereby.
20. The system of claim 19 wherein the processing unit is operative
to associate the ground speed with the corresponding laser beam
directional configuration of the at least three laser-based
apparatus, and operative to determine ground velocity using the
ground speeds and corresponding laser beam directions of the at
least three laser-based apparatus.
21. The system of claim 20 wherein the processing unit is operative
to determine ground velocity of the aircraft by a triangulation of
the ground speeds and corresponding laser beam directions of the at
least three laser-based apparatus.
22. The system of claim 18 wherein the processing unit is operative
to determine a ground speed of the aircraft for each of the at
least three laser-based apparatus based on a ratio of a difference
over a sum of the first and second signals corresponding
thereto.
23. The system of claim 18 wherein the laser source of each of the
at least three laser-based apparatus is autonomously operative to
periodically generate laser pulses; and wherein the light detection
means of each of the at least three laser-based apparatus is
operative to generate the first and second signals corresponding to
each received laser beam reflection.
24. The system of claim 23 wherein each laser-based apparatus
includes means for generating a pulse signal representative of a
start of each laser pulse period thereof.
25. The system of claim 24 wherein the processing unit is operative
to determine altitude of the aircraft based on the pulse signal and
at least one of the first and second signals of at least one of the
at least three laser-based apparatus.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention is directed to avionic systems for
measuring above ground level (AGL) altitude and ground velocity of
an aircraft in general, and more particularly, to combined laser
measurement apparatus for measuring both above ground level (AGL)
altitude and ground velocity of the aircraft.
[0002] Today's military and commercial aircraft desire more precise
measurements of aircraft position and position information. Ground
based RADAR systems and Global Positioning Systems (GPS) allow for
precise positioning of an aircraft in latitude and longitude
desirable for air traffic control, aircraft separation, and
navigation. However, precise AGL measurements are often difficult
to achieve with such systems, especially for applications in which
precise placement above the ground is needed. New levels of
precision for altitude or AGL measurements, on the order of +/-6
inches (15 cm), for example, are often required for flight profiles
ranging from hover, to nap of the earth (NOE) flight, and
autonomous landing. Current aircraft altimeter systems generally
can not achieve these precise measurements.
[0003] Recently, laser-based altimeters have been proposed for use
on-board aircraft. This laser altimeter technology presents a
significant advancement over radar altimeters as the ground
registered data contains a higher level of resolution due to the
narrow beam of the laser. However, the laser altimeters do pose
certain concerns when applied to aircraft, especially with regard
to the volume of the instrument attributed to the large number of
optical elements contained therein. Another concern is directed to
the ruggedness of the instrument in an aircraft flight environment.
The optical elements of the laser altimeter are generally mounted
on an optical bench, adjusted to be precisely aligned with respect
to each other and secured in place. But, because of the vibration,
shock and wide temperature variation encountered in aircraft
flight, the optical elements have a tendency to become misaligned
over time and thus, require constant maintenance. In bi-static
laser altimeter configurations, back scattering of laser beam
transmissions into a telescope portion is an additional
concern.
[0004] A laser altimeter which overcomes the aforementioned
concerns of laser altimeters by providing a compact laser altimeter
which improves upon size, ruggedness and maintenance of the
instrument is described in the co-pending U.S. patent application
Ser. No. 10/386,334, filed Mar. 11, 2003, entitled "Compact Laser
Altimeter System" and assigned to the same assignee as the instant
application.
[0005] Laser systems have also been proposed for use on-board the
aircraft in measuring the ground velocity thereof. These ground
velocity laser systems propose to use the backscattering of laser
emissions off of the ground, similar to laser altimeters, to
measure the ground velocity. However, such laser based ground
velocity measurement systems usually have more stringent optical
alignment concerns than those for the laser altimeters described
above.
[0006] The present invention overcomes the aforementioned concerns
by integrating the capability of measuring ground velocity into a
laser altimeter system, such as the system described in the
co-pending patent application Ser. No. 10/386,334, for example, to
effect a laser based system for measuring both AGL altitude and
ground velocity in a common instrument. Through use of common
optical and signal processing elements, the resulting combined
instrument maintains substantially the features of small size,
ruggedness and maintenance of the laser altimeter of co-pending
application Ser. No. 10/386,334.
SUMMARY OF THE INVENTION
[0007] In accordance with one aspect of the present invention,
combined laser-based apparatus for determining both altitude and
ground velocity of an aircraft comprises: a laser source for
emitting pulsed laser beams substantially at a predetermined
wavelength over a first optical path; a plurality of first optical
elements for directing the laser beams from the first optical path
to a second optical path which exits the first optical elements; a
plurality of second optical elements configured to form a telescope
with a predetermined field of view, the second optical path and
telescope field of view being fixedly co-aligned; an optical
scanner disposed in the second optical path for directing the
second optical path and telescope field of view to desired ground
positions while maintaining the co-alignment thereof; said
telescope for receiving from the desired ground positions Doppler
wavelength shifted reflections of the pulsed laser beams within the
field of view thereof and directing the received ground reflections
substantially over a third optical path; an optical filter element
disposed in the third optical path for separating the ground
reflections of the third optical path into first and second
portions that are dependent on the Doppler wavelength shift of the
ground reflections; and processing means for determining altitude
and ground velocity of the aircraft based on the first and second
portions.
[0008] In accordance with another aspect of the present invention,
laser-based apparatus for generating signals for use in determining
both altitude and ground velocity of an aircraft comprises: a laser
source for emitting pulsed laser beams substantially at a
predetermined wavelength over a first optical path; a plurality of
first optical elements for directing the laser beams from the first
optical path to a second optical path which exits the first optical
elements; a plurality of second optical elements configured to form
a telescope with a predetermined field of view, the second optical
path and telescope field of view being fixedly co-aligned; the
telescope for receiving Doppler wavelength shifted reflections of
the pulsed laser beams within the field of view thereof and
directing the received reflections substantially over a third
optical path; an optical filter element disposed in the third
optical path for separating the reflections of the third optical
path into first and second portions that are dependent on the
Doppler wavelength shift of the reflections; and light detection
means for receiving the first and second portions and generating
first and second signals representative of the first and second
portions, respectively.
[0009] In accordance with a further aspect of the present
invention, A distributed laser-based system for use on-board an
aircraft for determining both altitude and ground velocity of the
aircraft comprises: at least three laser-based measurement
apparatus for disposition at different locations on the aircraft,
each said apparatus comprising: a laser source for emitting pulsed
laser beams substantially at a predetermined wavelength over a
first optical path; a plurality of first optical elements for
directing the laser beams from the first optical path to a second
optical path which exits the first optical elements; a plurality of
second optical elements configured to form a telescope with a
predetermined field of view, the second optical path and telescope
field of view being fixedly co-aligned; the telescope for receiving
Doppler wavelength shifted reflections of the pulsed laser beams
within the field of view thereof and directing the received
reflections substantially over a third optical path; an optical
filter element disposed in the third optical path for separating
the reflections of the third optical path into first and second
portions that are dependent on the Doppler wavelength shift of the
reflections; and light detection means for receiving the first and
second portions and generating first and second signals
representative of the first and second portions, respectively; each
laser laser-based apparatus configurable to direct its co-aligned
second optical path and telescope field of view from the aircraft
to a different ground position from the other laser-based
apparatus; and a processing unit for receiving and processing the
first and second signals from the at least three laser-based
apparatus to determine both the altitude and ground velocity of the
aircraft.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is an illustration of apparatus for filter edge
detection of Doppler shifted wavelength ground return signals
suitable for use in an embodiment of the present invention.
[0011] FIG. 2 is a graph of a response characteristic curve of an
optical filter element suitable for use in the apparatus of FIG.
1.
[0012] FIG. 3 is an illustration of a combined laser altimeter and
ground speed measurement apparatus suitable for embodying the broad
principles of the present invention.
[0013] FIG. 4 is an illustration of apparatus for scanning both an
emitted laser beam and a field of view of a telescope in fixed
alignment suitable for use with the embodiment of FIG. 4.
[0014] FIG. 5 is a functional block diagram schematic of processing
electronics for computing a measurement of ground velocity suitable
for use in the present embodiment.
[0015] FIG. 6 is a block diagram of a processor for computing
altitude suitable for use in the present embodiment.
[0016] FIGS. 7 and 8 are side and top view illustrations of an
exemplary alternate non-scanning embodiment of the present
invention.
[0017] FIG. 9 is a block diagram schematic of exemplary processing
electronics suitable for use in the alternate non-scanning
embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The principle used in the present laser-based ground
velocity measurement apparatus embodiment is to emit pulses at a
predetermined wavelength and pulse repetition rate from a laser
source to a position on the ground and receive the laser light
backscattering off of the ground position. The backscattering of
laser light will be Doppler shifted in wavelength in proportion to
the portion of the ground speed of the aircraft along the
directional vector of the scan position of the emitted laser
pulses. Thus, by determining the Doppler shift of the emitted
wavelength and the scan position of the laser beam, the associated
ground speed projected along the directional vector may be
calculated. By triangulating these projected ground speed
measurements at three or more scan positions or angles, the
absolute ground velocity of the aircraft may be determined, without
requiring aircraft attitude information. The term "ground" as used
in this application shall mean any surface over which the aircraft
may fly including land, water, objects, . . . etc. and any
combinations thereof.
[0019] In the present embodiment, Doppler shift of wavelength is
determined by a process of filter edge detection. FIG. 1 is an
illustration of apparatus for filter edge detection of Doppler
shifted wavelength ground return signals suitable for use in the
present embodiment which will be described herein below in
connection with the illustration of FIG. 3. Referring to FIG. 1, an
optical filter element 10 which may be a dichroic beam splitter,
for example, is disposed in an optical path 12 of the Doppler
shifted ground return pulses.
[0020] The optical filter element 10 has a response characteristic
curve shown by way of example in the graph of FIG. 2. Note that the
response curve exhibits a very sharp cut off transmission response
with respective to wavelength. In the present embodiment, the
filter angle of the optical element 10 is tuned such that the
unshifted laser line .lambda..sub.0 is approximately half-way down
the cut-off edge. In this manner, a Doppler shift in wavelength of
.DELTA..lambda. will fall along the filter edge of the response
curve and effect a difference .DELTA.T in filter transmission of
the ground return pulses. Thus, as shown in the illustration of
FIG. 1, a first portion of the ground return pulses will be
transmitted or passed through the optical filter element 10 along
an optical path 14 based on the wavelength Doppler shift
.DELTA..lambda. thereof. A second or remaining portion of the
ground return pulses will be reflected by the optical element 10
along an optical path 16. There may be some loss of optical signal
in the filter element 10, but this should affect both the
transmitted and reflected portions in a measurable and repeatable
manner and thus can be accounted for in the data processing.
[0021] While a dichroic beam splitter is used as the optical filter
element 10 in the present embodiment, it is understood that other
optical filter elements may be used just as well. Examples of other
optical filter elements which may be used include: optical etalons
or Fabry-Perot cavities tuned by tilting; pressure, thermal
control, or piezo-electric drivers; bandpass filters with
sufficiently sharp edges; and atomic or molecular gas absorption
filters. If this system is embodied in a fiber-optic configuration,
a tunable fiber optic etalon may also be used, for example.
[0022] Light detectors 18 and 20 may be disposed in the paths 14
and 16 of the transmitted and reflected portions of the ground
return pulses, respectively, to receive and convert the respective
light pulses into electrical signals proportional thereto.
Accordingly, electrical signals S.sub.1 and S.sub.2 output from the
signal detectors 18 and 20 are proportional to the transmitted and
reflected portions, respectively, of the ground return pulses. By
taking the ratio of the sum and difference of the electrical
signals [(S.sub.2-S.sub.1)/(S.sub.1+S.sub.2)] while accounting for
losses in the filter element, ground return pulse amplitude effects
may be substantially eliminated in determining the wavelength
Doppler shift .DELTA..lambda. of the ground return pulses. Due to
differing filter edge shapes, this ratio expression may not bear a
linear relationship to the Doppler shift, but the exact
relationship can be determined for the particular filter used.
[0023] A combined laser altimeter and ground speed velocity
instrument suitable for embodying the principles of the present
embodiment is shown in the illustration of FIG. 3. The embodiment
of FIG. 3 uses many of the same optical elements and is configured
in much the same way as an embodiment described in the
above-referenced co-pending patent application Ser. No. 10/386,334
which is incorporated by reference herein for providing a more
detailed description thereof and other suitable exemplary
embodiments of a laser altimeter instrument for use in the present
invention. Referring to FIG. 3, a laser source 30 is disposed in a
laser transmitter assembly denoted by the dashed line block 32. The
laser source 30 may be a microlaser of the type manufactured by
Northrup Grumman Poly-Scientific, bearing model number ML0005, for
example. In the present embodiment, the microlaser 30 is a
passively Q switched autonomously operated microchip laser pumped
by a 950 micron diode to generate pulsed laser beams at a rate of
approximately 8-10K pulses per second (pps) and at a predetermined
wavelength, which may be approximately 1064 nanometers (nm), for
example.
[0024] The microlaser 30 may be contained in a TO-3 container or
can which may be fixedly secured to a wall of a housing the
instrument much in the same manner as described for the embodiment
of the incorporated co-pending application. The TO-3 can also
includes a windowed top surface 34 from which to emit the pulsed
laser beams over a first optical path 36. It is understood that the
specific pulse repetition rate and wavelength of the microlaser 30
are provided merely by way of example, and that other rates and
wavelengths may be used just as well without deviating from the
broad principles of the present invention. For example, for eye
safe operation, a laser emitting at a wavelength of approximately
1.5 microns may be chosen.
[0025] Fixedly supported in a compact configuration within an
emission cavity of the instrument is a plurality of first optical
elements for directing the laser beams from the first optical path
36 to a second optical path 38 which exits the housing of the
instrument through an exit aperture at 40. The plurality of first
optical elements may comprise a band pass or blocking filter
optical element 42 disposed in proximity to the windowed surface 34
of the microlaser 30 along the optical path 36. The optical element
42 may cover substantially the entire emission cavity opening so as
to block substantially the laser pump diode light and other
wavelengths of light outside of a predetermined bandwidth around
the predetermined wavelength of the laser beam from entering the
emission cavity. To minimize optical feedback that may cause laser
instabilities and to minimize the heat load on the laser chip,
optical element 42 may be disposed at an angle to the optical path
36 so that the surface thereof does not reflect light directly back
into the laser source 30.
[0026] Another first optical element of the plurality may be a
collimating lens 44 disposed along the first optical path 36 down
stream of the filter element 42 for collimating and preventing
further divergence of the laser beams along the path 36.
Collimating lens 44 may be disposed along path 36 so as to match
the laser beam divergence to a field of view of a telescope portion
of the altimeter for optimum efficiency as will become better
understood from the description below. While the lens 44 and filter
42 are provided in the present embodiment, it is understood that
due to the laser selected and the compactness of the overall
configuration, one or both of the lens 44 and filter 42 may not be
used in some applications.
[0027] To render the compact configuration of first optical
elements, it is understood that the emission beam path or optical
train of the transmitter assembly may take various shapes. In the
present embodiment, the beam path is shaped into a vertical "Z"
with the elements 42 and 44 on a top level and the exit aperture 40
disposed at a bottom level. A vertical channel of the assembly
cavity connects the top and bottom levels. Two fold mirrors 46 and
48 are included in the plurality of first optical elements and
disposed at the vertical channel to direct the first optical path
36 from the top level to the bottom level and to move the beam
close to the receiving telescope portion to minimize the range at
which the telescope field of view and the laser spot start to
overlap. The fold mirror 46 is disposed at the top level and the
other fold mirror 48 is disposed at the bottom level. Accordingly,
the combination of fold mirrors 46 and 48 direct the first optical
path 36 to the second optical path 38 which exits the housing 10 at
aperture 40. One of the fold mirrors 46 or 48 comprises mirror
apparatus which is fixedly adjustable for directing the second
optical path 38 along a desired optical path as will become more
evident from the following description. Preferably, the top fold
mirror 46 is the adjustable mirror, but it is understood that that
either fold mirror 46 or 48 may be used for adjustment purposes or
both mirrors may be adjustable along the independent axes.
[0028] Thus, all of the first optical elements are fixedly
supported and not movable in the emission cavity of the instrument,
except for the adjustable mirror of either fold mirror 46 or 48,
and even such mirror apparatus is lockable in place once properly
adjusted. The top level of the emission cavity may extend slightly
beyond the vertical channel for locating a light detector 50, which
may be a photo-diode, for example. In this embodiment, the fold
mirror 46 is configured to pass a small portion of the pulsed laser
beams for detection by the light detector 50 which converts the
detected laser pulses into electrical signals for use as start
pulses for time-of flight calculations as will become more evident
for the description found herein below.
[0029] The instrument housing may further include another cavity
for containing processing electronics for the laser altimeter and
ground speed velocity measurements much in the same manner as
described in the incorporated co-pending application. Such
processing electronics may be implemented on one or more printed
circuit (PC) boards, for example. The light detector 50 may be
coupled to the electronics in the electronics cavity for providing
the start pulses for time-of-flight and ranging calculations
thereby. Alternatively, a light detector diode may be embodied in
the TO-3 can of the microlaser 30 for detecting and providing laser
start pulses to the processing electronics via an electrical
coupling thereto. If a triggerable pulsed laser is used, the
trigger signal may also serve as the timing start pulse. It is
understood that these techniques for generating trigger or start
pulses are provided by way of example and that any method used will
depend on available space and the particular optical system
design.
[0030] A telescope portion 52 comprising a plurality of second
optical elements is included in another hollow cavity of the
instrument with an entrance aperture at 54 much in the same manner
as described in the incorporated co-pending application. The
plurality of second optical elements are fixedly disposed and
configured within the hollow cavity to form a telescope with a
predetermined field of view which is preferably fixed. The
telescope portion 52 is operative to receive at the entrance
aperture 54 reflections of the pulsed laser beams from the ground
position within the field of view thereof and to focus the received
reflections substantially to a focal point 56. The telescope
portion 52 includes a band pass filter optical element 58 disposed
at the entrance aperture 54 for passing received wavelengths of
light solely within a predetermined bandwidth around the
predetermined wavelength .lambda..sub.0 of the pulsed laser beams.
Thus, the filter optical element 58 minimizes background light
interference from the outside environment from entering the
telescope cavity. In addition, the field of view of the telescope
may have to be minimized to further reduce interference from
background solar radiation, for example. In some applications, a
clear window may be disposed at aperture 54 to seal and protect the
telescope from scratches and outside contamination; however, the
filter optical element 58 could be mounted in such a way to serve
the same purpose.
[0031] To form the telescope, the telescope portion 52 includes a
convex or converging lens 60 disposed in proximity to the entrance
aperture 54. In the present embodiment, the telescope lens 60 is
configured to have a predetermined focal length, which may be
approximately 150 millimeters (mm), for example, for focusing the
received reflections from the entrance aperture 54 to the focal
point 56, which falls within the telescope cavity. A fold mirror 62
may be fixedly disposed within the telescope cavity downstream of
the focal point 56 to reflect the received light rays illustrated
by the arrowed lines along a different optical path 64. If the
optical elements of the telescope portion 52 were to be used solely
for AGL altitude measurements, then a single light detector would
be disposed in the path 64 for receiving the light reflections off
of the ground. The present embodiment combines the AGL altitude
measurements with ground velocity measurements, and thus includes
additional optical elements for this purpose.
[0032] One of the additional optical elements of the telescope
portion 52 is a recollimating lens 66 disposed between the focal
point 56 and fold mirror 62 to recollimate the expanding light
reflections from focal point 56 prior to being reflected by the
mirror 62. Accordingly, the light reflected by the mirror 62 along
path 64 is substantially collimated. Another of the additional
optical elements is a tilt-tuned etalon 68 disposed in the optical
path 64. The etalon element 68 operates as the optical filter
element 10 described in connection with the embodiment of FIGS. 1
and 2 and may be tilt-tuned so that the wavelength .lambda..sub.0
falls mid-way along the sharp cut-off filter edge of the response
curve as described supra. Thus, a portion of the ground reflected
light will be transmitted through the etalon element 68 and be
refocused by a lens 70 to a light detector 72 much the same as
described for the embodiment of FIGS. 1 and 2.
[0033] Likewise, the remaining portion of the ground reflected
light (absent that lost in the filter element itself) will be
reflected by the etalon element 68 back to the fold mirror 62 along
path 64. From mirror 62, the remaining portion of the ground
reflected light is redirected by the mirror 62 back to the lens 70
wherein it is refocused to another light detector 74 much the same
as described for the embodiment of FIGS. 1 and 2. Both of the light
detectors 72 and 74 may be avalanche photo-diodes operative to
convert the received light pulse into an electrical signal
representative thereof. The outputs of the light detectors 72 and
74 may be coupled to the processing electronics in the electronics
cavity for use in both ground velocity and altitude ranging
calculations thereby as will become better understood from the
description infra.
[0034] Moreover, while the emission, electronics and telescope
cavities are provided in a common housing in the present
embodiment, it is understood that such cavities may be provided in
separate housings in an alternate embodiment. Such housings may be
sections of a common housing in yet another embodiment. In any
event, the common denominator for all such embodiments of the
combined laser-based instrument is to render the unit compact and
rugged for use in an aircraft flight environment. The present
embodiment of the instrument may have overall exemplary dimensions
in length L, width W and depth D of approximately 7.5 inches or 19
cm, 3.5 inches or 8.75 cm, and 3.5 inches or 8.75 cm, respectively.
In addition, while the aforementioned additional elements are
employed with the laser-based AGL altitude measurement embodiment
of FIG. 3, it is understood they may also be implemented in other
laser-based AGL altitude measurement embodiments, like those
described in the incorporated co-pending application referenced
herein above, for example, without deviating from the broad
principles of the present invention.
[0035] The emission and telescope cavities, whether in the same
housing or separate housings, are fixedly secured in alignment with
respect to each other to permit the output optical path of the
pulsed laser beams (see darkened arrowed line 80 in FIG. 4) to be
fixedly co-aligned with the field of view of the telescope (see
dashed lines 82 in FIG. 4). Note that only one first optical
element of the plurality, like mirror 46, for example, is fixedly
adjustable for co-aligning the output optical path 80 with the
field of view 82. In the present embodiment, the emission and
telescope cavities may be machined in the common housing to align
the entrance and exit apertures respectively thereof in proximity
to each other. It is preferable to have the apertures 40 and 54 as
close as possible to each other. The exit aperture 40 may be offset
slightly behind or in back of the entrance aperture 54 to avoid any
direct backscattering of the transmitted laser beams into the
entrance aperture 54 and telescope cavity. In addition, a flat
window may be disposed at the exit aperture 40 for sealing the
emission cavity from the outside environment. Also, this window may
be tilted with respect to the plane of the exit aperture 40 to
avoid reflections from the laser beams from traveling back down the
transmitting optical path into the laser, possibly causing laser
instabilities thereby. In addition, laser light may be reflected
from the tilted window to a photodiode as another technique for
generating the timing start pulses as described herein above.
[0036] FIG. 4 is an illustration of a scanner assembly suitable for
use in the present embodiment for scanning the laser beam 80 and
co-aligned field of view 82 of the telescope to different ground
positions while maintaining the co-alignment. Referring to FIG. 4,
a scanner mirror 84 is disposed in the path of the emitted laser
beam 80 and co-aligned field of view 82 at an appropriate quiescent
angle for projecting the laser beam 80 and co-aligned field of view
82 to a desired position on the ground. In the present embodiment,
the scanner mirror 84 may be rotated about an axis 86 to different
angles shown by the dashed lines 88 and 90 by a motor assembly (not
shown). At the different angles 88 and 90, the scanner mirror moves
the emitted laser beam 80 and co-aligned field of view 82 in
directions as shown by the arrowed lines 92 and 94, respectively,
to desired different ground positions. The mirror motor may be
controlled to direct the laser beam 80 and co-aligned field of view
82 to a plurality of desired ground positions by the processing
electronics as will become more evident from the following
description.
[0037] A functional block diagram schematic of processing
electronics for computing a measurement of ground velocity suitable
for use in the present embodiment is shown in FIG. 5. The
processing electronics may be disposed on one or more printed
circuit (PC) cards located in the electronics cavity of the
instrument, for example. Referring to FIG. 5, the light detectors
72 and 74 are represented by like reference numeral functional
blocks. The output of light detector 72 which is an electrical
pulse representative of the transmitted portion of the ground
reflected pulse is input to a threshold detect block 100. If the
electrical pulse amplitude of the transmitted signal portion is
greater than a predetermined threshold, the block 100 passes the
pulse signal to a peak detector block 102 which captures and
outputs the peak amplitude, denoted as S.sub.1, of the transmitted
pulse signal.
[0038] Similarly, the output of light detector 74 which is an
electrical pulse representative of the reflected portion of the
ground reflected pulse is input to a threshold detect block 104. If
the electrical pulse amplitude of the reflected signal portion is
greater than a predetermined threshold, the block 104 passes the
pulse signal to a peak detector block 106 which captures and
outputs the peak amplitude, denoted as S.sub.2, of the transmitted
pulse signal. The signals S.sub.1 and S.sub.2 may be input to a
processor 110, which may be a programmed microprocessor, for
example. Also, the processing electronics may include a detector
112 for detecting the position of the laser beam scan (vector) at
which each ground speed calculation is performed. The laser scan
position may be provided as a motor drive signal or provided by a
sensor located on the shaft of the scanner mirror, for example. In
the processor 110, a ratio R is computed by taking the difference
and sum of signals S.sub.1 and S.sub.2, and dividing the difference
by the sum as follows:
R=[(S.sub.2-S.sub.1)/(S.sub.1+S.sub.2)] (note that S.sub.1 and
S.sub.2 may have to be corrected for filter losses).
[0039] A look-up table may be provided in the processor 110 for
correlating ground speed with the above calculated ratio R. So, as
a new ground reflection pulse is received, S.sub.1 and S.sub.2 are
determined and the laser scan position is captured for that pulse.
The ratio R is calculated and the portion of the ground speed along
the directional scan vector of line-of-sight is accessed from the
look-up table based on the instant ratio R. This portion of the
ground speed and the associated scan position may be saved in
processor 110. Thereafter, the scanner mirror 84 (see FIG. 4) may
be directed by processor 110 over signal line 114, for example, to
project the laser beam 80 and co-aligned field of view 82 to a
different ground position and calculate the ground speed for this
new ground position in the same manner. The process will be
repeated by processor 110 until ground speeds are determined and
saved for at least three ground scan positions. Then, the processor
110 may perform a triangulation calculation, perhaps by matrix
inversion calculation, for example, on the three or more ground
speeds and associated scan positions to determine the instantaneous
velocity vector of the aircraft with respect to the ground, i.e.
ground velocity. This calculation may be expressed in an orthogonal
X, Y, and Z coordinate system by the following relationship:
1 X, Y, Z Ground LOS Rotation * Speed = Speed Matrix Vector
Vector
[0040] Accordingly, an inversion of the X, Y, and Z Rotation matrix
term multiplied by a matrix comprised of three different velocity
vector terms produces the ground speed vector matrix with respect
to the attitude of the aircraft and measuring instrument.
[0041] The amplitude of the ground speed vector, which is the
vehicle ground speed, may be output over signal line 116. Note that
the vehicle airspeed and attitude information is not required for
this calculation of ground speed. However, if these data are
accessible to the processor 110, absolute vehicle speed, direction
and sideslip may also be calculated by the processor 110.
[0042] The same processor 110 may be also programmed to perform an
AGL altitude calculation using the start or trigger signal from the
light detector 50 and signal S.sub.1 from detector 72, for example,
as shown in the block diagram of FIG. 6. A time-of-flight
measurement may be performed from the time between the start and
reception pulses from detectors 50 and 72, respectively, to
determine the range to the instantaneous ground position. The
processor may compensate the range for instantaneous laser scan
position using the signal from the detector 112 to determine the
actual AGL altitude which may be output over signal line 120. While
the signal S.sub.1 is used in the present embodiment for time of
flight determinations, it is understood that S.sub.2 or a
combination of S.sub.1 and S.sub.2 may be used just as well.
Accordingly, both AGL altitude and ground velocity may be
determined from common electrical signals and processing
electronics embodied in the combined laser-based apparatus.
[0043] In an alternate non-scanning embodiment to the scanning
embodiment described above in connection with FIG. 4, at least
three of the combined laser-based measurement apparatus, such as
that described for the embodiment of FIG. 3, for example, may be
distributed at different locations about the aircraft. Such a
non-scanning embodiment is shown in the illustrations of FIGS. 7
and 8 in which a helicopter 130 is used by way of example as the
aircraft. While a helicopter aircraft is used for the alternate
embodiment, it is understood that the multiple combined laser-based
measurement apparatus may just as well be mounted on other
aircraft, such as fixed wing aircraft, UAVs and PGMs, for
example.
[0044] Referring to FIGS. 7 and 8, four combined laser-based
measurement apparatus are mounted at different locations on the
aircraft 130. In the side view of FIG. 7, only two such apparatus
132 and 134 are shown mounted to the side of the aircraft by way of
example. The other two such apparatus may be mounted in similar
locations on the other side of the aircraft 130 such as shown in
the plan view of FIG. 8. The combined laser-based measurement
apparatus may be adjusted to project each of their co-aligned
emitted laser beam and field of view paths (see FIG. 4) 140, 142,
144 and 146 at predetermined vectors to corresponding ground
positions. While four measurement apparatus are shown by the
exemplary embodiment of FIGS. 7 and 8, it is understood that three
or more than four measurement apparatus may be mounted to the
aircraft for the non-scanning embodiment without deviating from the
broad principles of the present invention.
[0045] Each of the at least three measurement apparatus may include
a threshold detector and peak detector (see FIG. 5) for generating
the corresponding transmitted and reflected signals S.sub.1 and
S.sub.2, and a trigger light detector (e.g. 50, FIG. 3) for
generating the start or trigger signal T. Accordingly, the signals
T, S.sub.1 and S.sub.2 may be appropriately amplified, if desired,
and output from each of the measurement apparatus to a remotely
located central processing unit disposed on-board the aircraft. The
block diagram schematic of FIG. 9 exemplifies a non-scanning
distributed system for the aircraft in which three laser-based
measurement apparatus 132, 134 and 136 are mounted to different
locations thereof, such as shown by way of example in FIGS. 7 and
8, and output their respective signals T, S.sub.1 and S.sub.2 to a
remotely located on-board processing unit shown within the dashed
lines 150.
[0046] Referring to FIG. 9, the processing unit 150 of the present
embodiment may comprise a signal multiplexer 152, and
analog-to-digital converter (A/D) 154 and a programmed processor
unit 156. The signals T and S.sub.1 from each of the apparatus 132,
134 and 136 may be coupled over signal lines to a digital input
section (DI) of the processor unit 156. Such digital inputs may
either be polled or configured as program interrupts by the
processor 156 to identify a start and reception times for each of
the measurement apparatus 132, 134 and 136. In addition, signals
S.sub.1 and S.sub.2 from each of the apparatus 132, 134 and 136 may
be coupled over signal lines to inputs of the multiplexer 152. An
output 158 from the multiplexer 152 is coupled to an input of the
A/D 154 and output data lines 160 of the A/D 154 are coupled to a
data bus of the processor 156. The processor 156 may control the
operations of the multiplexer 152 and A/D 154 over control lines
162. In this embodiment, the peak detectors (see FIG. 5) of each of
the apparatus 132, 143 and 136 may include a sample-and-hold
circuit to hold the peak signals S.sub.1 and S.sub.2 of a current
interpulse period until the peak signals of the next interpulse
period are determined.
[0047] In a typical operation, the apparatus 132, 134 and 136 may
be autonomously operative to emit laser pulses periodically,
receive the ground reflections during the interpulse periods and
generate the signals T, S.sub.1 and S.sub.2 for each laser pulse
period. The processor unit 156 is programmed to detect the start of
each pulse period by monitoring the T signals and to compute the
time-of-flight by monitoring the signals S.sub.1 and/or S.sub.2,
for example, for each apparatus. Then, knowing the predetermined
vector of the laser beam path, the processor 156 may calculate the
AGL altitudes of the aircraft for the corresponding ground
positions of the apparatus 132, 134 and 136.
[0048] In addition, the processor 156 may be programmed to read in
the signals S.sub.1 and S.sub.2 from the apparatus 132, 134 and 136
for an interpulse period via control of the multiplexer 152 and A/D
154 and compute a ratio R (see FIG. 5) for each apparatus 132, 134
and 136 from the corresponding signals S.sub.1 and S.sub.2. Through
use of a look-up table, the processor 156 may determine a speed of
the aircraft for each computed ratio R corresponding to the
apparatus 132, 134 and 136. The vector path of each apparatus 132,
134 and 136 may be pre-programmed into the processor 156 for use in
combining with the corresponding calculated aircraft speed to
compute the ground velocity 164 of the aircraft (see FIG. 8),
preferably through a matrix inversion or a triangulation
calculation. In this manner, the distributed system of apparatus
132, 134 and 136 may determine both AGL altitude and ground
velocity of the aircraft using a common on-board processing unit
150.
[0049] While the present invention has been described above in
connection with one or more embodiments, it is understood that
these embodiments were presented by way of example. Accordingly,
the present invention should not be limited in any way by the
exemplary embodiments, but rather construed in breadth and broad
scope in accordance with the recitation of the appended claims.
* * * * *